1. Field of the Invention
The present invention generally relates to electrode foils and, more particularly, to a method for making electrode foils suitable for use as anodes in electrolytic capacitors.
2. Related Art
Electrolytic capacitors are compact, high voltage energy storage devices used in a variety of applications including, for example, in implantable medical devices (IMDs) such as implantable cardioverter defibrillators (ICDs). An ICD is a medical device that is implanted in a patient to monitor electrical activity of the patient's heart and to deliver appropriate electrical and/or drug therapy, as required. ICDs include, for example, pacemakers, cardioverters, defibrillators, and the like.
ICDs conventionally use electrolytic capacitors because such capacitors have a high energy density and can withstand a relatively high voltage. ICDs typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an ICD can utilize two 250 to 500 volt electrolytic capacitors in series to achieve a voltage of 500 to 1,000 volts.
One type of electrolytic capacitor includes an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. The electrolyte impregnated in the separator functions as the cathode in continuity with the cathode foil, while an oxide layer on the anode foil functions as the dielectric. The entire laminate may be rolled up into the form of a substantially cylindrical body, or wound roll that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, composing a planar, layered, stack structure of electrode materials with separators interposed therebetween.
Such capacitors used in ICD applications must typically be able to store a relatively large amount of energy (e.g., approximately 10-100 joules). Because the capacitance of an electrolytic capacitor increases with the surface area of its electrodes, one way to increase capacitance is to increase electrode (e.g., anode) surface area. But smaller size devices are desired for implantable devices such as ICDs. So the challenge is to increase surface area of the electrodes without increasing the physical size of the resulting capacitor.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. By electrolytically etching the foils that are used to form the anodes and cathodes of a capacitor, surface area can be increased without enlargement of the overall capacitor. As a result of this enlargement of the surface area, electrolytic capacitors can obtain a given capacity with a smaller volume than an electrolytic capacitor which utilizes a foil with a non-etched surface. Likewise, etched-foil capacitors of a given volume can obtain a higher capacitance compared to non-etched foil capacitors. Etching the foil increases the surface area of the foil by pitting an otherwise flat surface.
In a conventional electrolytic etching process, the surface area of a foil is increased by removing portions of the foil to create etch tunnels. The foil used for such etching is typically an etchable aluminum strip of high cubicity. The etch initiation and hence the gain or capacitance of the foil is the result of several variables, such as foil cubicity, thermal oxide on the foil, and the electrochemical reaction. As tunnel density (i.e., the number of tunnels per square centimeter) is increased, a corresponding enlargement of the overall surface area will occur. Larger surface area results in higher overall capacitance.
But etching comes at the cost of increased brittleness of the foil itself. In order to obtain high capacitance, foil material must be removed during the etching process to create the tunnels and increase the surface area. Additionally, a widening process may be used to open the tunnels to prevent clogging during later oxide formation. Both the etching and widening processes can remove as much as 50 to 60% of the foil material (e.g., aluminum) to create a desired etch degree of greater than 30 million tunnels per cm2. After formation of oxide during an aging process, the foil becomes even more brittle. This creates problems during further processing of the foil and assembly of the capacitor.
For example, a series of etched anode foils may be formed from a single aluminum sheet. After etching and otherwise processing the sheet, each anode is punched out by use of a mechanical die into an anode shape to conform to the geometry of the capacitor case. The more aluminum that is removed during etching and widening, the more difficult the foil is to punch from the sheet without creating cracks and loose particles.
After the anodes are punched by the mechanical die, the anodes are interleaved with paper and cathode layers and are assembled into stacks. But the edges of the brittle anodes can contain burrs and attached particles. And the burrs and particles can penetrate the paper layers and cause short circuits between the anode and cathode foils, compromising the quality and life of the capacitor. High potential tests performed to check the stacks for short circuit conditions prior to final assembly have shown that failures (i.e., short circuits) can occur in as many as 5 to 10% of the capacitor stacks depending on the brittleness of the anode foil.
Additionally after the punching process, the newly created edges of the anodes has exposed aluminum without a high-quality oxide formed thereon. After the assembly of the anodes, paper, and cathodes into a case, the capacitor case is sealed and filled with an electrolyte. Next, the capacitors are put through an aging process that reforms oxide on the edges and on any exposed cracks in the anodes. The oxide formed from the aging process is not as high quality as the oxide formed during the formation process. And the higher the ratio of edge surface to anode surface, the higher the potential is for increased leakage current.
In fabricating anode foils for use in an electrolytic capacitor having a multiple anode stack configuration, a tab extending from each anode foil is connected to tabs of adjacent anode foils of the stack to electrically connect the anodes together in parallel. To facilitate the electrical connection (e.g., by welding), the tabs are left un-etched. This can be accomplished using a mechanical mask to mask the tabs, and any other areas where etching is not desired during the etching process. For example, U.S. Pat. No. 5,660,737 to Elias et al. discloses use of a mechanical mask. Without the non-etched tab areas, welds will not appropriately form the connections between the anodes in a stack configuration. But the foils are susceptible to cracking along the etch/non-etch transition edge during the welding process. This can result in a tab detaching in the event of extreme crack propagation.
Another problem with using a mechanical mask in manufacturing anode foils for electrolytic capacitors is that the process is tedious, requiring operator involvement for mask alignment. This leads to production inefficiency. Further, since different capacitor products have different anode shapes, unique tab etch masks must be maintained for each particular product model, and current density must be optimized for each etch mask. This again leads to cost inefficiency and recurring costs as new products with different anode shapes are introduced.
What is needed is a high capacity anode foil and method of making such anode foil that overcomes deficiencies of known systems and methods.